1. Field of the Invention
The present invention is applicable in a field where nickel sulfide obtained from a low grade nickel oxide ore is used as a raw material for ferronickel smelting.
2. Description of the Related Art
Ferronickel, an alloy of nickel and iron, which is used as a raw material of stainless steel, or the like, is often produced by pyrometallurgy, in which a nickel oxide ore, such as laterite ore and garnierite ore, is introduced into a rotary kiln to be dried, and then introduced into a melting furnace, such as an electric furnace, with a reducing agent, such as coke, to be reduced and melted at a temperature of approximately 650 degrees C. In this case, a part of the oxide ore is dried in a kiln, and, without being reduced and melted, becomes a product as it is, as a nickel-iron mixed oxide in briquette form.
In recent years, high grade nickel oxide ores, such as garnierite ores, having not less than 2% nickel grade and having been conventionally used as raw materials, have been depleted, and accordingly there is a growing necessity to treat low grade nickel oxide ores, such as laterite ores, having approximately 1% to 2% or less nickel grade. However, when a nickel grade in an ore is low, a larger amount of ore needs to be treated in order to produce the same quantity of ferronickel and this causes scale-up of equipment. As a result, undesirable impacts, such as increase in investment related to production and increase in time and effort related to production, arise.
Furthermore, generally, nickel oxide ores often contain a lot of moisture, and therefore there was a problem that a quantity of fuel required for drying of nickel oxide ores by using a rotary kiln increases and accordingly cost increases. Hence, there has been conducted a study in which nickel in a low grade nickel oxide ore is condensed and reformed to a raw material having a high nickel grade, and the concentrated raw material is used to keep costs down.
For example, there is a method called HPAL as shown in Japanese Patent Application Laid-Open No. 2005-350766.
The method in Japanese Patent Application Laid-Open No. 2005-350766 is to provide a smelting method which is simple and efficient over a whole process, and achieved by simplification of a leaching step and a solid-liquid separation step, reduction of an amount of neutralizer consumption and an amount of precipitate in a neutralization step, and more efficient repeated use of water, in hydrometallurgy, which is based on high-temperature pressure leaching by to recover nickel from a nickel oxide ore.
Specifically, the smelting method comprises a leaching step, wherein sulfuric acid is added to a slurry of ore, followed by stirring treatment under a temperature of 220 to 280 degrees C. to form a leached slurry; a solid-liquid separation step, wherein multistage washing is performed for the leached slurry to obtain a leachate containing nickel and cobalt and a leaching residue; a neutralization step, wherein, while oxidation of the obtained leachate is controlled, calcium carbonate is added so that a pH is not more than 4, and thus a neutralized precipitate slurry containing trivalent iron and a mother liquor for recovering nickel are formed; and a sulfuration step, wherein hydrogen sulfide gas is blown into the mother liquor to form a sulfide containing nickel and cobalt, and a barren solution.
In other words, a nickel oxide ore is mixed with a sulfuric acid solution to be made into a slurry, then the slurry is heated in a high pressure vessel, such as an autoclave, and nickel and cobalt which are contained in the ore are leached into the sulfuric acid solution. Then, using a thickening apparatus, a filter press, or the like, the slurry is separated into a leachate and a residue. The obtained leachate is separated from impurities by pH adjustment, and a sulfurizing agent is added thereto to obtain a nickel-cobalt mixed sulfide.
The obtained mixed sulfide is transported to an existing hydrometallugical refining works, and leached using chlorine gas, sulfuric acid, or the like, and then cobalt is separated from nickel through steps, such as solvent extraction, and recovered as high-purity nickel metal and high-purity cobalt metal by a method, such as electrolytic winning.
Different from pyrometallurgy, which is a conventional common smelting method of nickel oxide ore, this method of Patent Literature 1 does not include dry type steps, such as a reduction step, a drying stage, etc., and use less energy, and thus is advantageous in terms of cost.
Furthermore, most of various kinds of impurities contained in nickel oxide ore, such as cobalt, iron, aluminum, manganese, zinc, chromium, magnesium, and silicon, other than nickel, were leached together with nickel from the ore by sulfuric acid, and therefore an important problem in a smelting process was separation and recovery of nickel by efficient means, but, by a neutralization step and a sulfuration step, the impurities can be effectively separated from nickel to obtain nickel sulfide and cobalt sulfide, each having less impurities. The method has been regarded as effective, for example, as a method for smelting a low grade nickel oxide ore with approximately 1 to 2% by mass nickel grade.
However, in Patent Literature 1, when the obtained sulfide is treated in the smelting works in which the above-mentioned oxide ore has been already treated, sulfur contained therein is oxidized, whereby a large amount of sulfur oxide (SOx) gas is emitted. Therefore, there is a problem that necessity to newly install equipment for exhaust gas treatment for sulfur oxide arises and a large amount of equipment investment is required.
Thus, it was not easy to smelt ferronickel from nickel sulfide.
Furthermore, Japanese Patent Application Laid-Open No. 2006-241529 (Page 1, Page 2, FIG. 1) describes a method comprising the steps of: obtaining a leachate containing nickel and cobalt by forming and fixing a residue by using iron contained in an ore as natron jarosite at the time of leaching by adding sulfuric acid to a nickel oxide ore in a upstream step; removing iron and aluminum by adding a neutralizer to the obtained leachate; obtaining an extracted residual solution containing nickel and a back-extraction solution containing cobalt, by treating the obtained neutralized solution by solvent extraction in which a monothiophosphinic acid compound is used as an extractant and thereby extracting cobalt; and obtaining a hydroxide by neutralizing each of the obtained extracted residual solution containing nickel and the obtained back-extraction solution containing cobalt, with alkali.
The obtained nickel hydroxide is washed using an alkali solution at a downstream step, whereby sulfur and chlorine each contained therein are removed, and then ferronickel is produced by a process including existing burning and reduction-dissolution steps.
However, when the method described in Japanese Patent Application Laid-Open No. 2006-241529 (Page 1, Page 2, FIG. 1) is used, the obtained nickel hydroxide often contains some sulfur resulting from involving sulfuric acid which is contained in the leachate. When burning and reduction-dissolution treatments are performed without removing the sulfur to produce nickel oxide, a problem that sulfur oxide generates arises, as is the case in the above-mentioned Japanese Patent Application Laid-Open No. 2005-350766.
It is difficult to remove the sulfur incidental to water of crystallization of the nickel hydroxide, only by washing in water, and therefore, as means to remove the sulfur incidental to the water of crystallization, Japanese Patent Application Laid-Open No. 2006-241529 (Page 1, Page 2, FIG. 1) describes a method for roasting nickel hydroxide at a temperature of approximately not less than 230 degrees C. to make nickel hydroxide and water of crystallization into sulfur and anhydrous nickel oxide.
However, even in the method of Japanese Patent Application Laid-Open No. 2006-241529 (Page 1, Page 2, FIG. 1), energy cost associated with the roasting at not less than 230 degrees C., and cost and time and effort necessary for treatment of exhaust gas cannot be ignored.
Furthermore, when a hydroxide is obtained by a hydrometallugical reaction as mentioned above, generally, fine particles are often formed, and when the fine particles are directly charged into a furnace in a downstream reduction melting step, hydroxide particles are dispersed or deposited on a wall of the furnace, whereby there is a concern that an operation trouble could arise or reaction efficiency could be not improved. Furthermore, the fine particles of nickel hydroxide tend to contain sulfur resulting from partially involving adhering water, water of crystallization, and the above-mentioned sulfuric acid solution before neutralization. When these hydrated particles are directly introduced into a high-temperature electric furnace, there are risks of causing a phreatic explosion due to water of crystallization and adhering water, generating corrosive gas by sulfur content and thereby promoting deterioration of equipment, or the like.
Therefore, in order to use nickel hydroxide as a raw material of ferronickel smelting, before reduction to ferronickel by an electric furnace, there is needed a complicated process requiring a treatment in which nickel hydroxide is introduced into a dry kiln for drying adhering water to be roasted and thereby changed into a nickel oxide, a treatment to remove sulfur before roasting, and the like. Thus, the production was costly.
In addition, most of nickel oxide ores often contain some cobalt accompanying nickel. The chemical property of cobalt is similar to that of nickel, and thus, when the method of Patent Literature 2 is used, cobalt contained in an ore coexists in nickel hydroxide without being separated from nickel. When this hydroxide is treated by an existing ferronickel producing process, cobalt is not recovered and losses are caused.
Japanese Patent Application Laid-Open No. 2007-77459 reports on a method for improving solid-liquid separation of a solid from a liquid each of which is generated by a neutralization step.
Specifically, the method comprises: a leaching step, wherein nickel or cobalt is leached from an oxide ore by using sulfuric acid to obtain a sulfuric acid leaching solution which contains nickel or cobalt, and a leaching residue; a reacting step, wherein pH adjustment is performed by reacting the sulfuric acid leaching solution containing the leaching residue with magnesium to obtain a reaction solution containing nickel or cobalt and a reaction residue containing iron; and a neutralization step, wherein the solution obtained in the upstream step is neutralized using a neutralizer to obtain a second neutralized solution containing nickel or cobalt and a second neutralized residue containing iron, and the method further comprises a preliminary neutralization step between the leaching step and the neutralization step, wherein a pH of the solution obtained at the upstream step is raised using an oxide ore.
However, according to the method of Japanese Patent Application Laid-Open No. 2007-77459, at a solvent extraction step in the latter stage using an organic solvent, nickel is maintained only at up to low concentration of approximately several g/L, therefore, a large amount of solution needs to be treated, whereby a scale of equipment is increased. Furthermore, since manganese contained in an oxide ore and existing in a leachate is extracted into an organic phase at the solvent extraction step, pH adjustment for separating manganese requires costs. Furthermore, there is a problem that an organic solvent accompanies a raffinate to cause losses, and cost increases accordingly, and thus the process is industrially disadvantageous.
As mentioned above, in the existing ferronickel smelting process, there has been required a method for producing nickel hydroxide which is effectively usable as a ferronickel raw material treatable by conventional methods.
At the same time, a method for effectively removing sulfur has been also needed.
An object of the present invention is to produce ferronickel from a nickel sulfide and a mixed sulfide containing nickel and cobalt, the nickel sulfide and the mixed sulfide being obtained by hydrometallurgy of nickel oxide ore or obtained from scraps and products in process.